Thermo Physics 101 – Sweating in Löyly

I always wanted to know how much do I actually sweat in Sauna. Instead of Googling some generic answer, this mystery called for a science project.

Why do we sweat in sauna?

Very simple – the air temperature in a sauna is significantly higher than your body temperature. The air heats up your skin and muscles and expands the veins. The blood pressure decreases and your heart starts pumping blood faster so that you won’t faint. External heat and higher pulse together increase the body temperature.

Your system then tries to lower your body temperature back to 37°C. If you have enough water in your body, your system moves warm water on to your skin and that lowers the body temperature. The sweat is then supposed to evaporate but that’s where the Finns trick science. The evaporation won’t happen in a hot and humid sauna unless the temperature of the sweat on your skin raises above the dew point, which it won’t. Consequently, your skin temperature keeps going up to above 40°C and this will eventually increase your body temperature to maybe 37.5 – 38°C or even more.

So technically you have a fever-like condition in sauna. Some call it hyperthermia. This is another fascinating topic for another blog post but your body generally increases its temperature to ask you to calm down (a few other things happen too). This might explain why you feel relaxed in a sauna and will sleep better after. We can dive deeper on this later.

In any event, you are supposed to get hot in sauna and that will make you sweat.

Have you always sweat wrong?

As also explained in my previous Generating Perfect Löyly blog post, the main idea in sauna is to make you sweat. Some people firmly believe that the hotter the sauna, the better. While I understand that hotter sauna makes you sweat more, I still suggest that whether the sauna temperature is 70°C or 90°C does not really matter. This is because the steam that hits your body is anyway about 100°C regardless of how warm the sauna is. If the steam is hotter than the air, it feels about the same on your skin. However, the hotter the sauna, the drier the air so a 90°C sauna, especially an electric one, can be really uncomfortable.

If your sauna is above 100c, then the steam actually cools your sauna down (maybe) but let’s not go there. The main point is to use water in a sauna because the steam sensation feels better than hot air alone. The steam also increases humidity and makes the air more breathable because it accelerates ventilation, which takes both carbon dioxide and sweat smell out faster and replaces that bad air with fresh air. This is probably why some saunagoers feel tired is a dry sauna but will feel better once the löyly happens.

So my choice is always to heat up my electric sauna to about 65-75°C and then generate a lot of steam. When I add water on the stove, I get really sweaty immediately. Or do I?

What is dew point and why it matters?

Wikipedia says: “The dew point is the temperature to which air must be cooled to become saturated with water vapor. When cooled further, the airborne water vapor will condense to form liquid water. When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.”

So, if the dew point is, say 10°C in the air and the grass on your backyard is 5°C warm, water from air will condense on the grass. Similarly, if you bring a cold water bottle to the sauna, the bottle stars sweating because the bottle temperature is lower than the dew point in the air. This same works with a bottle of beer, by the way.

Dew point can be calculated if you know the air temperature and relative humidity. The easiest way to calculate is to Google “dew point calculator”. For reference, dew point is 41°C , if the air is 70°C and relative humidity is 25%.

Why this matters? It matters because löyly significantly increases humidity in your sauna as explained in the About Sauna Temperature and Humidity blog post. That in turn, increases the dew point significantly. The diagrams below demonstrate this in practice: when you pour water on the stove, the absolute humidity increases sharply. Obviously, the relative humidity increases as well because the steam doesn’t dramatically change the temperature. In my experiment below, the dew point went above 60c on the head level, which means that a lot of steam condensates to water on your skin because your skin temperature is much less.

So it looks like you are sweating a lot in sauna but you are actually just drying down the sauna air. If you had 300 ml water on your skin in sauna, some percentage of that is condensed vapor from the air and some portion is sweat.

To the lab!

I don’t always wear a towel in a sauna but when I do, it gets really wet and heavy. It also seems that the floor on my sauna is getting sweaty. My project was to find out, how much do I actually sweat in a sauna.

To measure this I measured my weight and then took the dry weight of all the towels I had. I then placed towels on sauna benches and floors so that they would absorb all the sweat and condensed water. After the sauna, I would just weight my body and those wet towels again and take the water consumed during the sauna into consideration.

To make a perfectly believable study, I repeated my tests twice. The first time I only heated the sauna only to 50°C. It eventually warmed up to 65°C during the tests. The rocks were smoking hot. I did this to prove my point that the sauna temperature doesn’t matter as much if the rocks are about 200°C and you use plenty of water: vapor is still 100°C and when it hits your body you will sweat. The orange bar shows absolute humidity as I add water. Dew point increases as humidity and temperature increases. In the first session, the dew point exceeded 40°C and vapor started condensing on my skin. I did two 10-minute sessions with about 5-minute break.

After the second session I cooled down for about 5 minutes outside until I was not sweating anymore. Then I dried my curls and skin on those towels.

I drank 1.2 kg water (ok one water and two beers) and gained 600 grams weight. Thereby I sweat 600 grams or 600 milliliters. The towels gained 2.1 kg weight so they absorbed 1.5 kg moist sauna air. Therefore, only 29% of the “sweat” on my skin was actually sweat. 600 grams sweating is not a lot but this can be explained by unsually low temperature.

The next day I warmed the sauna a little bit more and started the session when it was 65c. I repeated two 10-minute sessions with about a five-minute break in between. Like the day before, there was about 70 grams water per cubic meter in the air but because the temperature was higher, the dew point was higher too.

This time, I dried myself immediately after the second session and weighted myself. This time, the result was 550g sweat and 1200g condensed water during the sauna session so about 31% of the “sweat” on my skin was actually sweat. However, I wanted to cool down and scale myself again 10 minutes after. Turns out that I sweat 500g more after the sauna session, which is quite a lot. It is easier for the body to sweat after sauna because the surrounding air is colder and less humid and the skin temperature decreases. So taking the cool down period into account, I could say that almost 50% of the water on my skin was sweat.

Other findings

I also measured the skin temperature on my arm during the session. It seemed that the temperature on the surface of the skin raised by about 5-7°C very quickly in a sauna. Some studies say that your skin temperature can increase by as much as 10°C in sauna. Fascinating data point.

Conclusions

  • I seem to sweat about 1 kg if I take two 10-minute sessions in a 70-75°C sauna and pour a normal amount of löyly.
  • Surprisingly lot of vapor condensates to water on my skin the sauna because the dew point rises to very high temperature in my barrel sauna.
  • My sauna keeps the steam in for quite long. If the löyly escaped from the vents quicker, the humidity would decrease quickly after löyly and less condensation would occur.
  • You probably won’t need this information for anything and I cannot give the reading time back to you. Sorrey, as the Canadians say.

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